In cement plants, a mixed and ground substance consisting of limestone, clay, silica stone, iron ore, and the like is burned at a high temperature in an SP kiln or in an NSP kiln, thereby producing a hydraulic clinker. The free lime content of the clinker exerts an influence on physical properties of cement, and accordingly, the factories produce the clinker while controlling the free lime content to fall within a certain range.
Conventionally, as the method for controlling the free lime content, a method of varying the mixing ratio of respective raw materials (adjusting chemical components of a mixture of raw materials), the amount of raw materials put into the kiln, the rotation speed of the kiln, the length of flames of a burner, the amount of kiln exhaust gas to be aspirated, and the like or a method of using a mineralizer is used.
For example, in the production method disclosed in PTL 1, the free lime content of the clinker and the like is controlled to be equal to or smaller than 0.5% by mass. Moreover, in the method disclosed in PTL 2, the free lime content and the fluorine content of cement are controlled such that they satisfy a certain relational expression. Furthermore, PTL 3 discloses a production method which makes it possible to decrease the cement clinker burning temperature without increasing the amount of fluorine by causing the burned clinker to contain one or more elements selected from a group consisting of fluorine, sulfur, chlorine, and bromine as well as one or more metal elements selected from a group consisting of group 3 to group 12 elements.
However, there is a limit to control the free lime content by the above methods, and in the current situation, a great fluctuation of the free lime content is unavoidable. When the free lime content fluctuates, the basic physical properties of cement (concrete), such as setting properties, strength, and fluidity, are influenced.